Not Applicable.
The present invention relates to electrochemical cells. In particular, the present invention relates generally to small electrochemical cells having solid cathodes, organic or solid polymer electrolytes comprising lithium salt solutes and aprotic, organic solvents. In a preferred aspect, this invention relates to lithium/graphite fluoride, especially lithium/carbon monofluoride electrochemical cells. While various electrochemical cell configurations are contemplated by the present invention, its primary application is coin cells or button cells.
It has long been a desire of the battery industry to have a very reliable, high rate performance, low impedance, high flash amperage electrochemical cell or battery that is capable of being stored and subsequently operated at a very wide range of temperatures. One of the electrochemical cell chemistries that have held great promise to meet that desire has been that of lithium graphite fluoride Li/(CF)n. A preferred form of the graphite fluoride cell chemistry is carbon monofluoride (Li/CFx). Lithium carbon monofluoride cells generally comprise a lithium anode, carbon monofluoride cathodes and a non-aqueous or organic electrolyte which generally includes a lithium salt.
Commonly owned U.S. Pat. No. 5,246,795 to Megahed et al. discloses a Li/CFx electrochemical cell which is structurally adapted for high temperature storage and use. Unfortunately, when stored and used at even wider temperature ranges, prior art Li/CFx electrochemical cell systems have not performed adequately, especially at low temperature. Specifically, after being exposed to temperatures in the range of xe2x88x9240xc2x0 C. to 110xc2x0 C. for long periods of time, the Li/CFX cells tend to provide insufficient current when called upon to do so e.g., by a remote tire pressure monitor interrogation system. Various other battery applications requiring such a wide operating temperature range would experience similarly suboptimal performance.
Accordingly, it is an object of the present invention to provide an electrochemical cell which may be stored and/or used for extended periods at a very wide range of temperatures.
It is a further object of this invention to provide a lithium graphite fluoride cell, especially a lithium carbon monofluoride electrochemical cell, which can be stored for lengthy periods of time even at relatively high temperatures and which is capable of delivering pulse currents of substantially greater amperage than previous systems.
It is yet a further object of the present invention is to provide a lithium solid cathode cell which can be produced according to known manufacturing techniques.
The above objects of the present invention will be more fully understood, and further objects and advantages will become apparent, from the following description of the invention.
Briefly, in one aspect, the present invention is an negative active material/organic electrolyte/active material cathode electrochemical cell which can be stored for long periods of time, at a wide range of temperatures, and which can deliver a surprisingly high flash amperage or pulse current. Anode materials generally comprise alkali metals and alkaline earth metals which are more completely described below. Cathode materials include, without limitation, graphite fluoride, carbon monofluoride, MnO2, CuS, FeS2 and include mixed cathode chemistries. Numerous other positive active material chemistries will occur to one skilled in this art in light of the present disclosure.
Thus, in one aspect, the present invention is a negative active material, organic electrolyte, positive active material electrochemical cell in which the cathode current collector surface and the adjacent cell can interior surface which are in conductive association with the cathode active material are substantially completely covered with a coating or layer comprising carbon powder. Generally speaking a carbon powder coating of this invention will include an adhesion-enhancing or promoting composition to enhance the adhesion between carbon coating and the current collector and adjacent cell can. Preferred adhesion promoters or binders of this invention are sodium silicate and potassium silicate.
In a further aspect, the present invention comprises an alkali metal/organic electrolyte/graphite fluoride electrochemical cell or battery, (especially a carbon monofluoride cell), in which the cathode current collector surface and adjacent cell can interior surface which are in conductive association or contact with the cathode active material are substantially completely covered with an adherent coating or layer comprising carbon powder.
In still a further aspect, the present invention includes an agent for counteracting an increase in internal impedance during short term high temperature storage and for improving low temperature high-rate pulse performance of a cell. By preventing the impedance from rising, the cells are able to operate at lower temperature, since any accompanying change in impedance does not rise to an unacceptable level.
In yet a further aspect, the present invention comprises a lithium/CFx electrochemical cell utilizing a carbon coated cathode current collector and can as is described above in which the cathode binder comprises all or substantially all fluoropolymer resin. Carbon powder coating (paint) containing silicate can confer improved corrosion resistance upon the cell. Corrosion resistance is particularly desirable in cells that contain fluoropolymer binder in the cathode, because the binder can increase corrosion in lithium cells after storage, notwithstanding the binder""s ability to confer improved high rate performance. Even without fluoropolymer binder in the cathode, the coating of the invention improves long term cell storage.
Yet a further aspect this invention is a method of making alkali metal or alkaline earth metal/(CF)n electrochemical cells, particularly Li/CFX cells, which can be stored at temperatures of as high as 110xc2x0 C. for many days and which can subsequently be discharged at a high pulse rate even at temperatures as low as xe2x88x9240xc2x0 C. Numerous other applications for the present invention will occur to one skilled in this art in light of the present disclosure taken in conjunction with the accompanying drawings.